It feels like I've finally brought the tank to life. It's amazing the difference it makes having a large shoal of one species! There's no more huddling under the filter outflow, now they're all over the tank, swimming in and out of plants. The flow doesn't seem to bother them now that they have confidence in numbers. This has taught me a lot about understocking!
I've also fallen in love with the lonely blue-eye who came in the threadfin bag by mistake. She is subtly different from the threadfins in appearance and behaviour, exhibiting slightly jerkier, more inquisitive movements. She will swim up to a patch of moss and peer at it intently as if searching for microfauna, while the threadfins don't tend to interact with the plants except when courting.
It turns out that I mis-identified her, she's not a furcatus but a gertrudae - a Spotted blue-eye. This is great because gertrudae are smaller and less aggressive than furcatus which means I can get her some friends without worrying too much about threatening the threadfins. I'm now looking forward to making this into a primarily rainbowfish species tank - although I haven't given up on the idea of adding a few pygmy cories or CPD's at a later date!
Meanwhile I have completely ruled out the idea of cherry barbs for this tank. I've learned that threadfins require very careful choosing of tankmates. They are very delicate eaters and I was unprepared for how small they need their food to be, or how easily they allow themselves to be out-competed at mealtimes. In retrospect I would not recommend pairing them with harlequin rasbora as I did because although totally peaceful the harlequins are far more vigorous when eating. Even the Spotted Blue-eye female is noticeably more adept at grabbing food than her threadfin companions. Additionally I think cherry barbs would be a bad idea with such long-finned fish, even though they are reputedly less nippy than other barbs it's not a risk I want to take.
Sorry for the rambling, I'm very excited about this tank at the moment!